The present invention relates to methods for attaching lead fingers to semiconductor dice and the structures formed thereby. More particularly, the present invention relates to methods for attaching lead fingers to semiconductor dice by using adhesive materials, and the structures formed from these methods.
The semiconductor industry has several goals for integrated circuit (IC) design and fabrication, such as increased performance, lower cost, increased miniaturization, and greater packaging density. One method of lowering the cost of designing and fabricating ICs is to reduce the amount of materials used, or use the materials more efficiently. One part of IC design and fabrication which inefficiently uses materials is processes for attaching or bonding semiconductor dice to a package or other connections such as lead frames. These processes include wirebonding, tape-automated-bonding connections, "lead-over-chip" ("LOC") connections, and a polymer dip chip used with a bumped die method. Another such process includes the process described in U.S. Pat. No. 5,585,282, the disclosure of which is incorporated herein by reference.
There are several methods for attaching a lead frame to a semiconductor die. One such method is the "lead-over-chip" ("LOC") method, such as those disclosed in U.S. Pat. Nos. 5,302,849, 5,548,160, and 5,286,679 ("the '679 patent"), the disclosures of which are incorporated herein by reference. The '679 patent discloses a LOC method which employs an adhesive material. The method described in the '679 patent applies a thermoplastic or thermoset adhesive to a semiconductor wafer. The adhesive layer is patterned during application to cover desired portions of the wafer by hot or cold screen/stencil printing or a dispensing process. After baking the adhesive layer on the semiconductor wafer to stabilize it, the individual dice are cut from the semiconductor wafer. During packaging, each adhesive-coated die is attached to the fingers of a lead frame by heating and pressing the fingers to the die. If the adhesive contains a thermoset material, a separate cure is then performed if the attach process does not, or cannot perform the cure. The method of the '679 patent unfortunately attaches the lead frame to the die inefficiently, i.e., it fails to achieve an adhesive material profile with enough upper surface area to efficiently attach the lead fingers.
In another LOC method, an adhesive tape (preferably insulative tape) is attached to an active surface of a semiconductor die and then lead fingers are attached to the adhesive tape. Although this method effectively attaches the fingers of the lead frame to the die, it is more expensive than using an adhesive material because the adhesive-coated tape costs more than the adhesive material. This adhesive tape method is also more expensive because of the fabrication steps required to cut individual tape segments from a larger sheet and material wasted when the tape segments are cut out. Further, the tape segments are often placed on a carrier film for transport to the die-attach site, raising both the cost and the complexity of the attachment process even more.
In yet another LOC method, an adhesive material is placed on the fingers of the lead frame rather than the semiconductor die. See, for example, co-pending U.S. application Ser. Nos. 08/906,673, 08/906,578, 08/709,182, 09/020,197, and 08/916,931, the disclosures of which are incorporated herein by reference. In one method, a liquid adhesive material is sprayed on the inverted attachment surfaces of the lead fingers. Some adhesive materials, however, may flow down the sides of the lead fingers and collect on the reverse surfaces of the lead fingers (e.g., the surfaces to which the bond wires will be attached). The adhesive material subsequently cures on these bond wire surfaces and can interfere with subsequent wire bonding, resulting in failure of the semiconductor component. The tendency for adhesives to flow from the lead finger attachment surfaces to the bond wire surfaces increases if the lead fingers are formed by a stamping process--which leaves a slight curvature, or rounding, of the edges of the lead fingers--rather than by an etching process. When this edge curvature is proximate the lead finger attachment surface, there is less resistance to the flow of the adhesive material and more adhesive material placed on the attachment surface consequently flows to the bond wire surface.
Finally, present methods of applying adhesive materials to a surface (whether of the semiconductor die or the lead finger) tend to waste the adhesive material. Spray application wastes adhesive material because not all of the sprayed adhesive material attaches to the target surface. Patterning the adhesive material on the semiconductor die results in substantial areas of the adhesive material of the pattern not being utilized. To obtain the combination of precise application with maximum material usage often makes the attachment process complex and difficult.